1. Field of the Invention
The present invention relates to a device that processes an image signal input from a color image-capturing element, a digital camera and a computer program product utilized in image signal processing, which, more specifically, achieve an improvement in the white balance performance.
2. Description of the Related Art
There are image signal processing devices such as electronic still cameras and video cameras in the known art that implement white balance adjustment in order to achieve faithful reproduction of white-color areas of the subject by processing image signals obtained through an image-capturing operation executed at a color image-capturing element (see Japanese Patent Publication No. 2997234, for instance).
FIG. 14 presents a control block diagram of an image signal processing circuit achieved in the related art. Now, a brief explanation is given on the white balance adjustment implemented in the related art in reference to FIG. 14. This camera includes an image-capturing device adopting the TTL system and an image signal processing device. A subject image formed on a color image-capturing element 52 by a photographic lens 51 is converted to an electrical image signal by the color image-capturing element 52. A brightness signal Y is generated from the image signal at a brightness signal processing unit 53 and a low-frequency component YL of the brightness signal, a red-color signal R and a blue-color signal B are generated from the image signal at a chroma signal processing unit 54. The brightness signal Y is a signal achieved by mixing red R, green G and blue B at a ratio satisfying the expression below.Y=0.30 R+0.59 G+0.11 B  (1)
The red-color signal R is multiplied by a white balance gain Rgain for the red-color signal and, as a result, a red-color signal R′ is generated at an R gain control unit 55. The blue-color signal B is multiplied by a white balance gain Bgain for the blue-color signal and, as a result, a blue-color signal B′ is generated at a B gain control unit 56. Next, a color difference signal R−Y is generated at a matrix amplifier 57 based upon the red-color signal R′ and the low-frequency component YL of the brightness signal. In addition, a color difference signal B−Y is generated at a matrix amplifier 58 based upon the blue-color signal B′ and the low-frequency component YL of the brightness signal. The color difference signals are expressed asR−Y=0.70 R−0.59 G−0.11 B  (2)B−Y=0.89 B−0.59 G−0.30 R  (3)These color different signals R−Y and B−Y are provided to a compression processing unit 59 where they are compressed in the JPEG format before they are recorded into a recording medium.
Averaging units 60 and 61 respectively ascertain the averages of the color difference signals R−Y and B−Y for the entire image plane. A control voltage calculation unit 62 calculates white balance gains Rgain and Bgain that will set the levels of the average signals to 0, i.e., the white balance gains Rgain and Bgain that satisfy R=B=G. As described above, the individual red-color signals R and blue-color signals B are respectively multiplied by the white balance gains Rgain and Bgain for a white balance adjustment.
However, there is a problem with the image signal processing device in the related art in that an accurate white balance adjustment cannot be achieved when photographing a scene with a subject having a chromatic color occupying a wide area of the photographic image plane, since the white balance adjustment operation is subjected to the predominant influence of the color of the subject on the photographic scene.
An object of the present invention is to achieve a good white balance even when a subject having a chromatic color takes up a wide area of the photographic image plane.